Page 32 - Fiber Bragg Gratings

Chapter 2

Photosensitivity and

Photosensitization of

Optical Fibers

We have seen in the last chapter that optical fibers have very good optical
properties for light transmission. Electronic absorptions that lead to atten-
uation are in the deep UV wavelength regime, and the molecular vibra-
tions are far removed from the optical fiber transmission windows of
interest to telecommunications. We have briefly considered the possible
link between the change in absorption and the effect on the refractive
index. Another possibility for the refractive index change is via an electro-
optic nonlinearity. However, the symmetry properties of glass prohibit
the electro-optic effect [1]. If there is an electro-optic contribution to the
changes in the refractive index as a result of exposure to UV radiation,
then an internal order would have to be created. This chapter considers
aspects of defects connected with photosensitivity and techniques for pho-
tosensitization of optical fibers. We briefly compare in Section 2.1 the
electro-optic effect [2] and how this may be invoked in glass. This aspect
has recently received considerable interest worldwide but, as already
stated, will not be studied in this book in any detail. Section 2.2 introduces
some of the defects that are linked to the UV-induced change in refractive
index of glass. The hot debate on defects has continued for a number of
years and there are a vast number of "subtleties" with regards to the
same nominal defect state, as well as pathways to achieving transforma-
tions from one state to the other. Some of the defects cannot be detected

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